Sample positioning apparatus

ABSTRACT

A sample positioning stage for positioning a sample to be inspected relative to an optical inspection device, including a first generally planar body on which a sample can be carried and a second body directly coupled to the first body via at least one rigid bearing and at least one resiliently compliant bearing provided between the first and second bodies. The first and second bodies are preloaded against each other in a dimension substantially parallel to the plane of the first generally planar body via the bearings. The stage also includes a motorized drive system operable to drive the first and second bodies relative to each other in the first plane toward a demanded relative position received from a position input device, and a position sensing device on at least one of the first and second bodies, for providing a measure of the relative position of the first and second bodies.

The present invention relates to a sample positioning stage, such as astage used to position a sample for inspection by an optical inspectionapparatus, for instance a microscope or spectroscope.

Sample positioning stages can be used, for example, to position samplesrelative to microscopes and spectroscopes, and can comprise a plate ontowhich a sample to be inspected can be placed. A generally planar plateis typically mounted on at least one carriage so that the plate can moverelative to the carriage in at least one substantially horizontal degreeof freedom. The plate and carriage typically have cooperating bearingmembers so that the plate and the carriage can be moved relative to eachother in at least one degree of freedom under the control of a motorizeddrive mechanism. The plate or the carriage can be coupled to a furthercarriage via bearings which facilitates movement of the plate in anotherdegree of freedom. The accuracy and repeatability of positioning of theplate is important in order to enable accuracy and repeatable positionof samples on the plate relative to an inspection device, for instance.

It is known to use motors for driving the plate and carriage of a samplepositioning stage relative to each other in response to signals receivedfrom an electronic input device. The use of a motor for driving a stagecan be advantageous for many reasons. For example, they enable theautomation of stage movement (e.g. a sample to be inspected or can beautomatically be placed in a plurality of different positions such thata montage of the sample can be obtained). The use of motors can alsoprovide for more accurate positioning of the stage. Motors can alsofacilitate stage movement when it is not possible for an operator to beclose to the stage.

It can be important to ensure that there is little or no play betweenthe bearings of the plate and the carriage; that is little or no freedomof movement between the bearings of the plate and carriage in dimensionsother than that in which the plate and carriage are intended to move.This is particularly the case when the plate and carriage are heldtogether only by way of the interaction of the cooperating bearingmembers. It can also be important to ensure that the force between thecooperating bearing members does not become too large so as to make itdifficult to move the plate and carriage relative to each other; thiscould result in too much load being put on the drive mechanism or evencause the bearing members to jam.

These undesirable situations can arise due to manufacturinginaccuracies. It is known to provide adjustable bearings, the positionof which can be set prior to operation of the stage for example throughthe use of adjustment screws, so as to avoid the above mentionedsituations. It is also known to provide spring loaded bearings betweenthe plate and carriage which compensate for any non-uniformity in thestage and thereby avoid the need for adjustable bearings.

However, it has been found that stages with spring loaded bearings canbe less accurate and provide less repeatable movement than those inwhich all of the bearings are rigid.

The present invention relates to improvements in sample positioningstages, and in particular provides a sample positioning stage havingrigid and compliant bearings extending and biased between the moveablebodies of the stage, in which a motorized drive system is configured toact on the bodies near to the rigid bearing, and in which a positionmeasurement device for providing a measure of the relative position ofthe bodies is positioned closer to the rigid bearing than theresiliently compliant bearing.

Accordingly, in a first aspect, the invention provides a samplepositioning stage for positioning a sample to be inspected relative toan optical inspection device, the stage comprising: a first generallyplanar body on which a sample to be inspected can be carried; a secondbody directly coupled to the first body via at least one rigid bearingand at least one resiliently compliant bearing provided between thefirst and second bodies, the at least one rigid bearing and the at leastone resiliently compliant bearing being arranged generally opposite eachother, and configured such that the first and second bodies arepreloaded against each other in a dimension substantially parallel tothe plane of the first generally planar body via the bearings, and suchthat movement of the first body relative to the second body isconstrained to a first plane that is substantially parallel to the planeof the first body via the bearings; a motorized drive system operable todrive the first and second bodies relative to each other in the firstplane toward a demanded relative position received from a position inputdevice, in which the motorized drive system imparts its driving force onthe stage closer to the at least one rigid bearing than the at least oneresiliently compliant bearing; and a position sensing device on at leastone of the first and second bodies closer to the at least one rigidbearing than the at least one resiliently compliant bearing, forproviding a measure of the relative position of the first and secondbodies.

It is an advantage that configuring the motorized drive system such thatit acts on the bodies closer to the rigid bearings than the compliantbearings, and positioning the position sensing device closer to therigid bearing than the compliant bearings improves the accuracy andrepeatability of movement of the stage.

The relative movement of the first and second bodies could beconstrained via the bearings to a first degree of freedom which liessubstantially parallel to the plane of the first body. The first degreeof freedom can be a linear degree of freedom. Optionally, the degree offreedom can be a rotational degree of freedom.

Suitable bearings for use with the present invention (for use witheither or both the rigid bearing or the resiliently compliant bearing)include sliding bearings and roller bearings such as cross-rollerbearings. For example, at least one of the rigid and resilientlycompliant bearings can comprise roller bearings, such as ball bearings,being located between tracks on the first and second bodies.

Preferably, each of the at least one rigid bearing and at least oneresiliently compliant bearing comprise a first bearing formation on thefirst body and a first bearing formation on the second body whichcooperate to form the bearing. The bearing formation can comprise abearing part which facilitates relative movement between the bodies inthe first plane. The bearing formation can also comprise a mount viawhich the bearing part is mounted to one of the first and second bodies.For example, the first and second bearing formations can comprise atrack on one of the first and second bodies and a runner on the otherfor cooperating with the track. As will be understood, the track can beany suitable feature on one of the bodies which defines a path which therunner on the other of the bodies is configured to follow. The track canbe profiled, for example toothed. In this case the runner could becog-shaped such that the track and runner engage like a rack and pinionmechanism. Preferably the track is smooth. In this case the runner couldbe configured to engage the track such that it can slide along thetrack. Optionally, the runner could be a roller configured to roll alongthe track. For instance, the runner could be a wheel.

As will be understood, different combinations of the types of bearingscould be used for the at least one rigid bearing and at least oneresiliently compressible bearing. For instance, the at least one rigidbearing could be a sliding bearing and the at least one resilientlycompressible bearing could be a roller bearing. Preferably, the sametype of bearing is used for each of the at least one rigid bearing andthe at least one resiliently compressible bearing. Preferably, the atleast one rigid bearing and the at least one resiliently compressiblebearing each comprise a track and a roller for rolling along the track.Preferably, the track for the at least one rigid bearing and the trackfor the at least one resiliently compliant bearing are provided on thesame body, for example the first body. Accordingly, preferably, therunner for the at least one rigid bearing and the runner for the atleast one resiliently compliant bearing are provided on the same body,for example the second body. In embodiments in which the first degree offreedom is rotary, the runner of the at least one rigid bearing couldengage the same track as the runner of the at least one resilientlycompliant bearing.

A track can be provided by at least one recess in the first or secondbody. A track can be provided by at least one projection on the first orsecond body. The profile of a track taken perpendicular to the length ofthe track can be curved, for example at least part elliptical,preferably at least part circular, for example semi-circular. A trackcan be provided as an integral part of one of the first and secondbodies. For example, a track and the body on which it is provided can beformed as one piece. A track can be provided as a separate piece whichis mounted on one of the first and second bodies. For example, one ofthe first and second bodies can comprise a mounting for a track.Preferably the mounting supports a track substantially along its length.

Movement of the first body relative to the second body is constrained toa first plane that is substantially parallel to the plane of the firstbody via the bearings. Accordingly, the first and second bodies areconstrained from moving relative to each other in a dimensionperpendicular to the plane of the first body via the bearings. Thebearing formations on the first and second bodies which cooperate toform the at least one rigid bearing and at least one resilientlycompliant bearing could comprise cooperating formations which preventthe first and second bodies from moving relative to each other in adimension perpendicular to the plane of the first body. For instance,the bearing formations could have inter-engaging formations, such as aprojection and groove arrangement, which prevents the first and secondbodies from moving relative to each other in a dimension perpendicularto the plane of the first body. Optionally, the bearing formations onthe first and second bodies which cooperate to form the at least onerigid bearing and at least one resiliently compliant bearing could beconfigured to frictionally engage under the preload so as to prevent thefirst and second bodies from moving relative to each other in adimension perpendicular to the plane of the first body via the bearings.

As will be understood, the at least one rigid bearing could permitrelative movement of the first and second bodies in the first degree offreedom but be sufficiently rigid so as to prevent relative movement inall other degrees of freedom. The bearing formations provided on thefirst and second bodies which provide the rigid bearing could besufficiently rigid such that their position cannot move relative to thebody on which they are provided. In embodiments in which the bearingformation comprises a bearing part and a mount, preferably the bearingpart and the mount are sufficiently rigid such that their positioncannot move relative to the body on which they are provided. Forexample, in embodiments in which the rigid bearing comprises a track onone of the bodies and runner on the other, preferably both the track andthe runner are fixed such that their position cannot move relative tothe body on which they are provided. Of course, as will be understood,in embodiments in which the runner is a roller, for instance a wheel,the roller will be able to rotate about its bearing relative to the bodyon which it is provided. In embodiments in which a part extends betweenthe formations on the first and second bodies, for example a rollerbearing extending between a track on each of the bodies, the part couldalso be sufficiently rigid, and shaped and sized so as to permitrelative movement of the first and second bodies in the first firstdegree of freedom but prevent relative movement in all other degrees offreedom.

In contrast, the at least one resiliently compliant bearing could permitrelative movement of the first and second bodies in the first degree offreedom, and also be resiliently compliant in at least one other degreeof freedom. This enables the resiliently compliant bearing to compensatefor any non-uniformity in the shape and size of the first and secondbodies and/or the bearing formations of the resiliently compliantbearing, as they move relative to each other. Preferably the resilientlycompliant bearing is not compliant in the first degree of freedom. Inparticular, preferably the resiliently compliant bearing is onlycompliant in a dimension that extends substantially perpendicular to thefirst degree of freedom at the bearing. Preferably, the resilientlycompliant bearing is rigid in a dimension that extends substantiallyperpendicular to the plane of the first body. Preferably the resilientlycompliant bearing is resiliently compliant in a dimension that extendsalong a plane which contains both the at least one resiliently compliantbearing and the at least one rigid bearing.

As will be understood, either or both of the bearing formations providedon the first and second bodies can provide the resilient compliance. Forinstance, in embodiments in which the bearing is provided by a track onone of the bodies and a runner on the other, the track side can beresiliently compliant. Preferably, the runner side is resilientlycompliant. Optionally, both the track side and the runner side can beresiliently compliant. In any case, preferably, the resilientlycompliant formation is resiliently compliant in a dimension whichextends substantially perpendicular to the length of the track at therunner, so as to bias the runner and track together. Preferably, theresiliently compliant formation is rigid in all other degrees offreedom.

As will also be understood, in these cases, a bearing part itself can beresiliently compliant. For example, the track itself can be resilientlycompliant. For example, in embodiments in which the bearing part is arunner, the runner itself can be resiliently compliant. Further still,in embodiments in which a bearing formation comprises a roller bearing,the roller bearing itself can be resiliently compliant. Optionally, inembodiments in which a bearing formation comprises a bearing part and amount, the mount can be resiliently compliant. Preferably the bearingpart itself is rigid. For instance, preferably the track is mounted tothe body on which it is provided by a resiliently compliant mount. Inembodiments in which a bearing formation comprises a runner, preferablythe runner is mounted to the body on which it is provided by aresiliently compliant mount.

The resiliently compliant mount can be provided as a separate componentto the body. For instance, the resiliently compliant mount can be aspring device, such as a coil spring, which acts between the bearingformation and the body on which the bearing formation is provided. Inthis case, the bearing formation can be coupled to the body by at leastone arm so as to support the bearing formation. The arm can be hinged tothe body so as to allow the arm and bearing formation to move relativeto the body. Optionally, the arm could be deformable so as to allow thearm and bearing formation to move relative to the body.

Preferably, the resiliently compliant mount and the body are formed asone piece. More preferably, the resiliently compliant mount and the bodyare formed from the same material. This can reduce the complexity of thestage as it reduces the number of components needed. This can decreasethe manufacturing cost, and also can improve the reliability of thestage.

The resiliently compliant mount can be configured to have at least onepredetermined point of weakness which is configured to deform when thefirst and second bodies are assembled together. This enables thedeformation of the resiliently compliant mount to be controlled in apredetermined way.

Preferably, the resiliently compliant mount comprises at least a firstarm which extends between the body and the bearing formation so as tosuspend the bearing member from the body. The first arm can beconfigured to deform at the point at which the first arm meets the body.Accordingly, the first arm will maintain its shape along substantiallyits entire length, but will bend at its joint with the body.

Preferably, the resiliently compliant mount further comprises at least asecond arm which extends between the body and the bearing formation soas to suspend the bearing formation from the body. Especiallypreferably, the resiliently compliant mount further comprises at least athird arm which extends between the body and the bearing formation so asto suspend the bearing formation from the body. Most preferably, theresiliently compliant mount further comprises at least a fourth armwhich extends between the body and the bearing formation so as tosuspend the bearing formation from the body.

When there is only a single rigid bearing and a single resilientlycompliant bearing, then preferably they are arranged directly oppositeeach other. Preferably the rigid bearing and resiliently compliantbearing are arranged such that a straight line connecting the bearingsextends perpendicular to the first degree of freedom at the bearings.However, as will be understood, the at least one rigid bearing and atleast one resiliently compliant bearing need not necessarily be arrangeddirectly opposite each other. As will be understood, when there are aplurality of rigid bearings, and/or a plurality of resiliently compliantbearings, then there can be various preferred arrangement of thebearings between the first and second bodies.

Preferably, the at least one rigid bearing and at least one resilientlycompliant bearing are arranged substantially opposite each other suchthat the direction of the net bias force which the at least one rigidbearing is under is substantially opposite to the direction of the netbias force which the at least one resiliently compliant bearing isunder.

Preferably, there is provided a bias mechanism configured to bias the atleast one rigid bearing and at least one compliant bearing into bearingengagement between the first and second bodies. As will be understood,because the at least one rigid bearing and at least one resilientlycompliant bearing are arranged generally opposite each other, the biasmechanism will tend to bias the at least one rigid bearing and the atleast one resiliently compliant bearing generally away from, or toward,each other. The bias mechanism could comprise a resiliently deformabledevice, for instance a spring. The resiliently deformable device couldbe provided by one or both of the first and second bodies. For instance,one or both of the first and second bodies could comprise at least firstand second portions corresponding to the at least one rigid bearing andat least one resiliently compliant bearing and the bias mechanism couldbe configured to bias the portions so as to bias the bearings intobearing engagement.

Preferably, the bias force is provided by the at least one resilientlycompliant bearing. Accordingly, preferably the at least one resilientlycompliant bearing biases the at least one rigid bearing and at least oneresiliently compliant bearing into bearing engagement. In this case theat least one resiliently compliant bearing can have a normalconfiguration which it is biased towards. Preferably, the stageapparatus is configured such that the at least one resiliently compliantbearing is forced away from its normal configuration against its bias.Preferably, the at least one resiliently compliant bearing is forcedaway from its normal configuration against its bias by way of theinteraction between the first and second bodies.

As described above, the at least one resiliently compliant bearing cancomprise a bearing part which facilitates relative movement between thebodies in the first degree of freedom, and the bearing part can bemounted to one of the first and second bodies via a mount. The bearingpart of the at least one resiliently compliant bearing can beresiliently compliant. Optionally, both the bearing part and the mountof the at least one resiliently complaint bearing can be resilientlycompliant. Preferably, the mount of the at least one resilientlycompliant bearing is resiliently compliant. The stage apparatus can beconfigured such that the resiliently compliant mount is forced away fromits normal configuration against its bias such that the resilientlycomplaint mount is in a biased state. Preferably, the resilientlycompliant mount is forced away from its normal configuration against itsbias by way of the interaction between the first and second bodies, andthe bearings.

As will be understood, when the at least one rigid bearing and at leastone resiliently compliant bearing are in bearing engagement theyfacilitate relative movement between the first and second bodies. Forexample, in embodiments in which the rigid bearing comprises a track anda runner as discussed in more detail below, the rigid bearing is inbearing engagement when the track and runner are biased into engagementwith each other such that the runner can be guided along the track.Preferably, when the at least one rigid bearing and at least oneresiliently compliant bearing are in engagement there is substantiallyno freedom of movement in the bearings other than in the first degree offreedom.

Preferably, the at least one rigid bearing and the at least oneresiliently compliant bearing are arranged such that, in embodiments inwhich the at least one resiliently compliant bearing provides the biasforce, each of the at least one resiliently compliant bearing isconfigured to bias at least one of the rigid bearings in bearingengagement. Preferably, the at least one rigid bearing and the at leastone resiliently compliant bearing are arranged such that the net forceprovided by the at least one resiliently compliant bearing biases eachof the rigid bearings into bearing engagement. Preferably, the at leastone resiliently compliant bearing and the at least one rigid bearing areconfigured such that the force provided by the bias of the resilientlycompliant bearings on each of the rigid bearings is substantially thesame.

As will be understood, there can be provided at least two resilientlycompliant bearings.

The motorized drive system can impart its driving force (so as to causerelative movement between the first and second bodies in the firstdegree of freedom) on any part of the stage close to the at least onerigid bearing. The motorized drive system could act on the at least onerigid bearing. The motorized drive system could act on the first body.The motorized drive system could act on the second body. In this case,the body on which the motorized drive system does not act could be heldagainst movement in the first degree of freedom. Optionally, themotorized drive system could act on the first and the second bodies tocause relative movement. The motorized drive system could be fixedrelative to one of the bodies in at least the first degree of freedom,and configured to act on the other of the bodies so as to cause relativemovement in the first degree of freedom.

Preferably the ratio of the smallest straight line distance between i)the point at which the motorized drive system imparts its driving forceon the first body or the second body and its closest at least oneresiliently compliant bearing; and ii) the point at which the motorizeddrive system imparts its driving force on the first body or the secondbody and its closest at least one rigid bearing is more than 1:1, morepreferably more than 2:1, especially preferably more than 5:1, forexample most preferably more than 10:1.

In embodiments in which the first degree of freedom is linear,preferably the ratio of the distance between i) the point at which themotorized drive system imparts its driving force on the first body orthe second body and its closest at least one resiliently compliantbearing; and ii) the point at which the motorized drive system impartsits driving force on the first body or the second body and its closestat least one rigid bearing, each taken in a dimension perpendicular tothe first degree of freedom is more than 1:1, more preferably more than2:1, especially preferably more than 5:1, for example most preferablymore than 10:1.

In embodiments in which the first degree of freedom is a rotationaldegree of freedom, preferably the at least one rigid bearing and the atleast one resiliently compliant bearing are arranged such that all ofthe at least one resiliently compliant bearings are on one side of aplane which wholly contains the axis of rotation and which extendsperpendicularly to the net bias provided by the resiliently compliantbearings, and so that all of the at least one rigid bearings are on theother side of the plane. In this case, preferably the motorized drivesystem acts on the body on the side of the plane on which all of the atleast one rigid bearings are located.

In embodiments in which the first degree of freedom is a linear degreeof freedom, preferably all of the at least one resiliently compliantbearings are provided on one side of a plane extending parallel to thefirst degree of freedom, and all of the at least one rigid bearings areprovided on the other side of the plane extending parallel to the firstdegree of freedom. In particular, preferably all of the at least oneresiliently compliant bearings are positioned toward one side of thestage and all of the at least one rigid bearings are position toward anopposing side of the stage in a direction perpendicular to the firstdegree of freedom. In these cases, preferably the motorized drive systemacts on the side of the stage on which all of the at least one rigidbearings are located.

The motorized drive system can comprise a controller for controlling amotorized drive unit. Accordingly, in this case, the motorized driveunit imparts the driving force on the first body or the second body, andthe controller controls the operation of the motorized drive unit inresponse to demanded relative positions received from a position inputdevice.

Suitable motorized drive units for use with the present inventioninclude belt drives, lead screws, rack and pinion drives, linear drivesand direct drives. In a particularly preferred embodiment the motorizeddrive unit comprises a motor body from which a drive shaft extends forengagement with a motor track, such that rotation of the drive shaft bythe motor body causes relative movement between the drive shaft and themotor track along the motor track's extent. Preferably, the motorizeddrive unit is configured such that rotation of the drive shaft causesrelative movement between the drive shaft and the motor track in adimension perpendicular to the rotational axis of the drive shaft.Preferably, the drive shaft is configured to frictionally engage afriction rod.

Preferably, in embodiments in which the bearing comprises a track, themotorized drive system acts on the track to cause relative movement.This is advantageous because it enables the stage to be made morecompact and lightweight. It also reduces the amount of torque exerted onthe bodies by the motor about the at least one rigid bearing. Forinstance, in embodiments in which the motorized drive system comprises amotor having a drive shaft for engaging a friction rod, preferably thedrive shaft frictionally engages the track. Accordingly, the track canbe the friction rod.

Preferably, the force applied by the motorized drive system on the trackis substantially in the same direction as the first degree of freedom ofrelative movement of the first and second bodies at the point themotorized drive system acts on the track. More preferably, the motorizeddrive system acts on the bearing surface of the track. As will beunderstood, the bearing surface can be the surface which a cooperatingrunner engages. Preferably there is at least one point of contactbetween the motorized drive system and the bearing surface of the track.For instance, there can be two points of contact between the motorizeddrive system and the bearing surface of the track. Preferably, thedirection of the force applied by the motorized drive system on thetrack extends substantially parallel to a bearing contact line whichextends substantially parallel to the extent of the track and containsthe point of contact between the runner and the track. More preferably,the point at which the motorized drive system acts on the bearingsurface of the track lies on the bearing contact line. Accordingly, itcan be preferred that the direction of the force applied by themotorized drive system on the track substantially extends along thebearing contact line. As will be understood, in embodiments in which thetrack is straight, then preferably the direction of the force applied bythe motorized drive system on the track and the bearing contact line aresubstantially co-axial. As will also be understood, there can be morethan one bearing contact line, for instance, in embodiments in whichthere are at least two points of contact between a runner and the track.In these embodiments it can be preferred that the motorized drive systemacts on the track so as to apply a force that extends along at least twoof the at least two bearing contact lines.

There can be provided at least two rigid bearings provided between thefirst and second bodies. For instance, there can be provided at leasttwo rigid runners on one of the first and second bodies for engaging arigid track provided on the other. As will be understood, the rigidrunners can be spaced apart along the extent of the track. In this case,preferably the motorized drive system acts on the track at a pointbetween the at least two spaced apart rigid runners. For instance, inthe embodiment in which the motorized drive system comprises a driveshaft for frictionally engaging a friction rod, preferably the driveshaft engages the track at a point between the rigid runners.Preferably, the motorized drive system acts on the track at a pointapproximately midway between adjacent rigid runners.

The motorized drive system can be rigid in that the part of themotorized drive system which acts on the stage to cause relativemovement cannot move relative to the body on which motorized drivesystem is anchored in a first dimension perpendicular to the firstdegree of freedom at the point of contact. Preferably, the part of themotorized drive system which acts on the stage to cause relativemovement is resiliently compliant. Preferably, the part of the motorizeddrive system which acts on the stage to cause relative movement isresiliently compliant in a dimension which is wholly contained in acompliance plane which also wholly contains the dimension in which theat least one resiliently compliant bearing is compliant. Preferably, thepart which acts on the stage to cause relative movement is biased intothe body on which it acts. For example, in embodiments in which themotorized drive system comprises a drive shaft which engages a track,preferably there is provided a bias mechanism which biases the driveshaft into frictional engagement with the track. Preferably the biasmechanism is resiliently compliant. The bias mechanism can act betweenthe body which the motorized drive mechanism is fixed relative to in thefirst degree of freedom and the drive shaft.

Preferably the ratio of the smallest straight line distance between i)the point on the first or second body from which the position sensingdevice determines the relative position of the first and second body andits closest at least one resiliently compliant bearing; and ii) thepoint on the first or second body from which the position sensing devicedetermines the relative position of the first and second body and itsclosest at least one rigid bearing is more than 1:1, more preferablymore than 2:1, especially preferably more than 5:1, for example mostpreferably more than 10:1.

In embodiments in which the first degree of freedom is linear,preferably the ratio of the distance between i) the point on the firstor second body from which the position sensing device determines therelative position of the first and second body and its closest at leastone resiliently compliant bearing; and ii) the point on the first orsecond body from which the position sensing device determines therelative position of the first and second body and its closest at leastone rigid bearing, each taken in a dimension perpendicular to the firstdegree of freedom is more than 1:1, more preferably more than 2:1,especially preferably more than 5:1, for example most preferably morethan 10:1.

In embodiments in which the first degree of freedom is a rotationaldegree of freedom, preferably the at least one rigid bearing and the atleast one resiliently compliant bearing are arranged such that all ofthe at least one resiliently compliant bearings are on one side of aplane which wholly contains the axis of rotation and which extendsperpendicularly to the net bias provided by the resiliently compliantbearings, and so that all of the at least one rigid bearings are on theother side of the plane. In this case, preferably the position sensingdevice measures the relative position of the first and second bodies ata point on the side of the plane on which all of the at least one rigidbearings are located.

In embodiments in which the first degree of freedom is a linear degreeof freedom, preferably all of the at least one resiliently compliantbearings are provided on one side of a plane extending parallel to thefirst degree of freedom, and all of the at least one rigid bearings areprovided on the other side of the plane extending parallel to the firstdegree of freedom. In particular, preferably all of the at least oneresiliently compliant bearings are positioned toward one side of thestage and all of the at least one rigid bearings are position toward anopposing side of the stage in a direction perpendicular to the firstdegree of freedom. In these cases, preferably the position sensingdevice measures the relative position of the first and second bodies ata point on the side of the stage on which all of the at least one rigidbearings are located.

The position sensing device can comprise a scale on one of the first andsecond bodies and a scale reader on the other of the first and secondbodies. In this case, preferably, the scale and scale reader areposition closer to the at least one rigid bearing than the at least oneresiliently compliant bearing. In particular, preferably the scalereader reads the scale at a point closer to the at least one rigidbearing than the at least one resiliently compliant bearing.

Suitable scales include those having marks defining a pattern which canbe read by a readhead in order to determine relative movement betweenthem. For instance, the scale can be an incremental scale having scalemarks defining a periodic pattern which generates a periodic signal atthe readhead when relative movement between the scale and readhead takeplace. These periodic signals produce an up/down count which enablesdisplacement between the scale and the readhead to be determined. Forinstance, such a suitable scale is described in European PatentApplication no. 0207121, the entire content of which is incorporatedinto the specification of the present application by this reference. Thescale can have reference marks which are detectable by the readhead sothat it can determine the exact position of the readhead relative to thescale. For example, such a scale is disclosed in Published InternationalPatent Application WO 2005/124282, the entire content of which isincorporated into the specification of the present application by thisreference. Optionally, the scale can be an absolute scale which hasscale markings which enable the readhead to determine an exact absoluteposition relative to the scale without the need to incrementally countfrom a predetermined position. Such scales typically have scale markingswhich define unique position data. The data can be in the form of, forinstance, a pseudorandom sequence or discrete codewords. Such a scale isdisclosed in International Patent Application no. PCT/GB2002/001629, theentire content of which is incorporated into the specification of thepresent application by this reference.

Position data from the position sensing device could be used be used inorder to determine the relative position of the first and second bodies.This could be used for instance to provide a relative position readoutto a user via a user interface display. Optionally, the position datacould be used in a control algorithm to detect when the first and secondbodies are moving relative to each other. Furthermore, positioninformation from the position sensing device could be used by themotorized drive system, for instance by the motorized drive system'scontroller, as part of a servo loop in order to accurately drive thefirst and second bodies relative to each other to a demanded relativeposition.

Preferably, the first and second bodies are generally planar.Preferably, one of the first and second bodies extends between theother. Preferably, the bearings are provided towards opposing sides ofthe bodies. Preferably, one of the bodies is held within another body bythe bearings. In particular, preferably one of the bodies has first andsecond side walls depending from opposing sides of the body, betweenwhich the other body is held against relative movement in a dimensionperpendicular to the plane of the bodies by the bearings. Preferably,the first and second side walls are linear. Preferably, the first andsecond side walls are parallel to each other. Preferably, each of thefirst and second walls provides a track. Accordingly, preferably themating sides of the other body provide at least one runner for engagingthe corresponding track.

Preferably, at least a portion of each of the first and second bodies(and third body if present) permits the transmission of light. As willbe understood, light can be visible, infrared or ultraviolet light. Thisis advantageous as it enables the illumination of a sample placed on thefirst body. The portion can be a transparent or translucent part of thebody. Optionally, the portion can be an opening in the body. Preferably,in embodiments in which the first degree of freedom is linear,preferably the portion of at least one of the first and second bodies iselongate. The portion can be elongate in a dimension perpendicular tothe linear degree of freedom in which the first and second bodies canmove relative to each other. In embodiments in which the second body isdirectly coupled to a third body, preferably the second body's portionis elongate in a dimension parallel to the linear degree of freedom inwhich the second and third bodies can move relative to each other.

The stage apparatus can further comprise: a third body directly coupledto the second body via at least one rigid bearing and at least oneresiliently compliant bearing between the second and third bodies, theat least one rigid bearing and the at least one resiliently compliantbearing being arranged generally opposite each other and configured topreload the second and third bodies against each other so as toconstrain movement of the first body relative to the second body to asecond degree of freedom which lies in a plane that is substantiallyparallel to the plane of the first body and which extends substantiallyperpendicularly to the first degree of freedom. A second motorized drivesystem can be provided and configured to drive the first and secondbodies relative to each other along the second degree of freedom towarda demanded relative position received from a position input device.Preferably, the motorized drive system is configured to impart itsdriving force on the stage to cause the relative movement in the seconddegree of freedom closer to the at least one rigid bearing than the atleast one resiliently compliant bearing.

The stage can further comprise a second position sensing device on atleast one of the second and third bodies closer to the at least onerigid bearing than the at least one resiliently compliant bearing, forproviding a measure of the relative position of the second and thirdbodies.

The stage can be for use in an inspection apparatus, such as amicroscope or a spectroscope. Preferably, the stage is for use in highresolution systems. For example, the stage can be for use in a highresolution system in which the resolution of the positioning of thefirst body to the second body is at least four orders of magnitudehigher than the range though which the first and second bodies can moverelative to each other, more preferably at least five orders ofmagnitude, especially preferably at least six orders of magnitude.

Preferably the position input device is an electronic position inputdevice. Suitable electronic position input devices include a joystick,trackball or other device which a user can manipulate to input ademanded relative position. The electronic position input device can bea memory device which contains pre-stored demanded relative positions.Optionally, position input device is a processor unit, for instance ageneral purpose computer which can provide demanded relative positionsfrom a computer program. Accordingly, user can program a sequence ofpositions which the program runs through to control the position of thestage apparatus.

According to a second aspect of the invention, there is provided anoptical inspection apparatus comprising: an optical inspection device;and a sample positioning stage as claimed in any preceding claim forpositioning a sample to be inspected relative to the optical inspectiondevice.

According to a third aspect of the invention, there is provided a samplepositioning stage for a microscope apparatus, comprising: a firstgenerally planar body having first and second linear trackssubstantially opposing and parallel to each other, at least the firstlinear track being rigidly fixed relative to the first body; a secondbody directly coupled to the first body so that it can move relative tothe first body in a degree of freedom, the second body having at leastfirst and second rigidly mounted runners in bearing engagement with thefirst linear track at points spaced along the first linear track so asto define the degree of freedom, the second body also having at leastone resiliently compliant runner in bearing engagement with the secondlinear track, in which the runners and tracks constrain movement of thefirst body relative to the second body to a first plane that issubstantially parallel to the plane of the first body; a motorized drivesystem fixed relative to the second body in the degree of freedom andconfigured to act on the first linear track at a point between which thefirst and second rigidly mounted runners engage the first linear trackso as to drive the first and second bodies relative to each other alongthe degree of freedom toward a demanded relative position received froma position input device; and a position sensing device on at least oneof the first and second bodies provided on the side of the bodies thatis closer to the rigidly mounted runners than the resiliently compliantrunner, for providing a measure of the relative position of the firstand second bodies.

An embodiment of the invention will now be described with reference tothe accompanying drawings in which:

FIG. 1 is a perspective view of an optical inspection apparatus having asample positioning stage according to the present invention;

FIG. 2 is a side elevation view of the optical inspection apparatusshown in FIG. 1;

FIG. 3 is a perspective view of the plate of the sample positioningstage shown in FIG. 1;

FIG. 4 is a perspective underside view of the sample positioning stageshown in FIG. 1, and shows a first carriage and the plate;

FIGS. 5 a and 5 b are plan and perspective views of the carriage shownin FIG. 4;

FIG. 6 a is a perspective underside view of the sample positioning stageshown in FIG. 1, and shows the first and a second carriage and theplate;

FIG. 6 b is a perspective topside view of the second carriage inisolation;

FIG. 7 is a detail view of a drive mechanism mounted within the samplepositioning stage shown in FIG. 1;

FIGS. 8 a and 8 b are perspective views of the drive mechanism shown inFIG. 7;

FIG. 9 is a perspective view of a bearing member of the drive mechanismshown in FIG. 7;

FIG. 10 illustrates the deformation of the runner mounting of a carriageof the sample positioning stage;

FIG. 11 is a schematic diagram of a control system and input devicecoupled to a sample positioning stage according to the presentinvention;

FIG. 12 is a flow chart showing the method of operation of the positionmaintenance module of the control system shown in FIG. 11;

FIG. 13 is a flow chart showing the method of operation of thecollision/drag detection module of the control system shown in FIG. 11;

FIG. 14 is a perspective underside view of a second embodiment of asample positioning stage according to the invention;

FIGS. 15 a is schematic underside view of the second guide rod, driveshaft and bearing wheels of the first and second carriages shown inFIGS. 1 to 14;

FIG. 15 b is a schematic cross-sectional view of the drive shaft andguide rod shown in FIG. 15 a; and

FIG. 15 c is a schematic cross-sectional view of a bearing wheel andguide rod shown in FIG. 15 a.

Referring now to FIGS. 1 and 2 there is shown an optical inspectionapparatus 2 which comprises a sample positioning stage 4 (hereinafterreferred to as “stage”) and an optical inspection device in the form ofa microscope 6.

The microscope 6 comprises an objective lens 10, first 12 and second 14eye piece lenses, and an arm 16 which supports the sample positioningstage 4.

As will be understood, the optical inspection device need notnecessarily be a microscope, and can be any device suitable forexamining a sample 8 placed on the stage 4. For instance, theexamination device could be a spectroscope. Furthermore, it will beunderstood that there need not be an optical inspection device at all.For example, the stage 4 could be used to support a sample 8 which is tobe examined by the naked eye.

Referring in particular to FIGS. 2 to 5, the stage comprises a plate 18,a first carriage 20 and a second carriage 22. As will be understood, thesample positioning stage 4 will typically be configured such that theplate 18 is oriented horizontally.

The second carriage 22 is fixed relative to the microscope 6, and inparticular is fixed relative to the objective lens 10, in the X and Ydimensions.

The plate 18 has an upper face 24 which is substantially planar andsubstantially rectangular in shape. In the embodiment described, aformation in the form of a recessed area 26 is provided for receiving asample 8 to be examined. Also provided is an aperture 27 through so thata light source (not shown) located below the plate 18 can illuminate asample 8 located in the recessed area 26. First 28 and second 30 skirtsdepend from opposing sides of the plate 18.

A handle 40, which is substantially cylindrical in shape, depends fromone corner of the plate 18. The handle 40 is placed on the side of theplate 18 that is distal to the arm 16. This aids accessibility of thehandle 40. A recess 42 is provided in the upper face 24 of the plate 18for receiving a thumb of a user. The handle 40 and recess 42 facilitategripping of the plate 18 by a user.

A first position measurement device is provided in the form of a firstscale 31 (a part of which is shown in FIG. 4) provided on the undersideof the plate 18, which can be read by a first readhead 33 (shown inFIGS. 5 a and 5 b) mounted on the first carriage 20. The first readhead33 is electrically connected to a control system (not shown) via a line(not shown) in cable 146, and can output a signal which can be used bythe control system to determine the position of the plate 18 relative tothe first carriage 20. A suitable scale is the scale sold under productnumber RGS40 available from Renishaw plc. A suitable readhead for is thereadhead sold under product number RGH34 readhead available fromRenishaw plc.

Referring to FIGS. 4, 5 and 6 a and 6 b, the first carriage 20 has abody 21 which is shaped and sized so that it is a snug fit between thefirst 28 and second 30 skirts of the plate 18.

The body 21 of the first carriage 20 has an elongate aperture 64, and aplurality of portions of reduced depth such as those indicated byreference numeral 65. The aperture 64 allows the passage of lightthrough the body 21 as described in more detail below, and the reduceddepth portions 65 reduce the weight of the body 21.

A second position measurement device is provided in the form of a secondscale 35 provided on the underside of the first carriage 20, which canbe read by a second readhead 37 mounted on the second carriage 22. Thesecond readhead 37 is electrically connected to the control system 200via a line (not shown) in cable 146, and can output a signal which canbe used by the control system 200 to determine the position of the firstcarriage 20 relative to the second carriage 22. A suitable scale is thescale sold under product number RGS40 available from Renishaw plc. Asuitable readhead for is the readhead sold under product number RGH34readhead available from Renishaw plc

Third 66 and fourth 68 skirts depend from the body 21, and extendbetween the first and second sides on which the first 44, second 46 andthird 48 wheels are mounted. The third skirt 66 has a third elongaterecess 70 extending along its length for receiving a third guide rod 72.The fourth skirt 68 has a fourth elongate recess 74 extending along itslength for receiving a fourth guide rod 76. The third 74 and fourth 78guide rods are held within the third 70 and fourth 74 elongate recesses.

First and second rigid bearings, generally indicated by 43 and 45, areprovided between the plate 18 and the first carriage 20 one side of thestage 4. A first resiliently compliant bearing generally indicated by47, is provided between the plate 18 and the first carriage 20 on theopposite side of the stage 4. The bearings 43, 45 and 47 facilitate therelative movement of the plate 18 and first carriage 20 in theX-dimension.

The bearings 43, 45 and 47 are provided by cooperating bearingformations provided on the plate 18 and carriage 20, as described inmore detail below.

The first skirt 28 has a first elongate recess 32 extending along itslength for receiving a first guide rod 34. The first guide rod 34provides the bearing part for the bearing formation on the plate 18 forthe first 43 and second 45 rigid bearings. The second skirt 30 has asecond elongate recess 36 extending along its length for receiving asecond guide rod 38. The second guide rod 38 provides the bearing partfor the bearing formation on the plate 18 for the first resilientlycompliant bearing 47.

The first carriage 20 has first 44 and second 46 rigid wheels positionedspaced apart on a first side of the first carriage 20. The first 44 andsecond 46 rigid wheels provide the bearing parts for the bearingformations on the first carriage 20 for the first 43 and second 45 rigidbearings. The first carriage 20 also has a rigid third wheel 48positioned on a second side of the first carriage 20, opposite to thefirst side. The rigid third wheel 48 is positioned along the length ofthe second side so that it lies midway between the first 44 and second48 rigid wheels on the first side. The third rigid wheel 48 provides thebearing part for the bearing formation on the first carriage 20 for thefirst resiliently compliant bearing 47.

The first 44 and second 46 rigid wheels are mounted within first 58 andsecond 60 circular recesses in the body 21 so that a portion of themextends beyond the boundary of the body 21. This is so that when thesample positioning stage is assembled as shown in FIG. 4, the first 44and second 46 wheels engage the first guide rod 34, thereby providingthe first 43 and second 45 rigid bearings. Likewise, the third rigidwheel 48 is mounted within a third circular recess 62 in the body 21 sothat a portion of the third wheel 48 extends beyond the boundary of thebody 21. When the sample positioning stage is assembled, the third rigidwheel 48 engages the second guide rod 38, thereby providing the firstresiliently compliant bearing 47.

The plate 18 and the first carriage 20 are configured so that when theyare assembled together, the force on the wheels on the first carriage 20and the respective guide rods on the examination plate is sufficientlylow that the first carriage 20 is free to move within the plate 18 alongthe guide rods, but is sufficiently high that there is no play betweenthe plate 18 and the first carriage 20 in all other dimensions.

The first 44, second 46 and third 48 rigid wheels each have circularapertures and the wheels are mounted by the apertures forming aninterference fit with first 52, second 54 and third 56 square pegswithin the first 58, second 60 and third 62 circular recesses. The useof circular apertures with square pegs enables an accurate and securemount between the wheels and the pegs. The first 44, second 46 and third48 wheels contain bearings which enable them to rotate relative to thepegs. The circumferential edge of the first 44, second 46 and third 48rigid wheels is grooved so that the wheels can partially wrap aroundtheir respective guide rod. This prevents the wheels, and hence thecarriage moving relative to the plate in the Z dimension, i.e.perpendicular to the plane of the plate 18.

The first 52 and second 54 pegs are mounted on the first carriage 20 sothat they cannot be moved relative to the body 21. In contrast, thethird peg 56 is mounted on the first carriage 20 so that it canresiliently move relative to the body 21 in a direction perpendicular tothe length of the second guide rod 38 of the plate 18 when the samplepositioning stage 4 is assembled.

In particular, the third peg 56 is mounted on a planar base 82 which isconnected to the body 21 by first 84, second 86, third 88 and fourth 90arms which define first 92, second 94 and third 96 apertures. The first84, second 86, third 88 and fourth 90 arms are resiliently deformableand allow the planar base 82, and so the peg 56 and the third wheel 48mounted on it, to move into the body 21 in the direction indicated byarrow A, on the application of a force on the planar based 82 in thedirection indicated by arrow A. The first 84, second 86, third 88 andfourth 90 arms are configured to deform along their length asillustrated in FIG. 10 (which shows an exaggeration of the amount thearms will actually deform in the described embodiment).

The stage 4 is configured such that when the plate 18 and first carriage20 are assembled together the first 84, second 86, third 88 and fourth90 arms are deformed. Accordingly, due to their resilience, the first84, second 86, third 88 and fourth 90 arms will bias the third rigidwheel 48 into the second guide rod 38. This in tutn will cause the first44 and second 46 rigid wheels to be biased into the first guide rod 34.

In the described embodiment, the stage 4 is configured so that the forceon the planar base 82 (caused by the force on the third wheel 48 by itsengagement with the second guide rod 38) is greater than the yieldstress of the first 84, second 86, third 88 and fourth 90 arms.Accordingly, the first 84, second 86, third 88 and fourth 90 arms areplastically deformed on assembly of the plate 18 and the first carriage20.

Referring to FIGS. 6 a, 6 b and 7, the second carriage 22 has a body 23which is shaped and sized so that it is a snug fit between the third 66and fourth 68 skirts of the first carriage 20. The second carriage 22 issubstantially identical to the first carriage 20, apart from that theshape of the aperture 67 (which allows the passage of light through thesecond carriage 22 as described in more detail below) is circular ratherthan elongate. This is possible because the second carriage 22 will befixed relative to a light source, whereas the first carriage 20 will beable to move relative to the first carriage 20 in the Y dimension.

Third and fourth rigid bearings, generally indicated by 81 and 83, areprovided between the first carriage 20 and the second carriage 22 on oneside of the stage 4. A second resiliently compliant bearing generallyindicated by 85, is provided between the first carriage 20 and thesecond carriage 22 on the side of the stage 4 opposite to that on whichthe third 81 and fourth 83 rigid bearings are provided. The bearings 81,83 and 85 facilitate the relative movement of the first carriage 20 andsecond carriage 22 in the Y-dimension.

The bearings 81, 83 and 85 are provided by cooperating bearingformations provided on the first carriage 20 and second carriage 22, asdescribed in more detail below.

The fourth guide rod 78 provided on the first carriage 20 provides thebearing part for the bearing formation on the first carriage for thethird 81 and fourth 83 rigid bearings. The third guide rod 72 providedon the first carriage 20 provides the bearing part for the bearingformation on the first carriage 20 for the second resiliently compliantbearing 85.

As with the first carriage 20, the second carriage 22 has first 77 andsecond 79 rigid wheels which are positioned spaced apart on a first sideof the second carriage 22. The first 77 and second 79 rigid wheelsprovide the bearing parts for the bearing formations on the secondcarriage 22 for the third 81 and second 83 rigid bearings. The secondcarriage 22 also has a third rigid wheel 80 positioned on a second sideof the second carriage 22, opposite to the first side. The rigid thirdwheel 80 is positioned along the length of the second side so that itlies midway between the first 77 and second 79 rigid wheels on the firstside. The rigid third wheel 80 provides the bearing part for the bearingformation on the second carriage 22 for the third resiliently compliantbearing 85.

The first carriage 20 and the second carriage 22 are configured so thatwhen they are assembled together, the force on the wheels on the secondcarriage 22 and the respective guide rods on the first carriage 20 issufficiently low that the second carriage 22 is free to move within thefirst carriage 20 along the guide rods, but is sufficiently high thatthere is no play between the first carriage 20 and the second carriage22 in the other dimensions.

The first 77 and second 79 rigid wheels are mounted on the secondcarriage 22 so that they cannot be moved relative to the body 23. Incontrast, the rigid third wheel 80 is mounted on the second carriage 22so that it can move relative to the body 23 in a direction perpendicularto the length of the fourth guide rod 72 of the plate 18 when the samplepositioning stage 4 is assembled.

In particular, the rigid third wheel 80 is mounted via a peg 87 which inturn is mounted on a planar base 89 which is connected to the body 23 byfirst 91, second 93, third 95 and fourth 97 arms which define first 99,second 101 and third 103 apertures. The first 91, second 93, third 95and fourth 97 arms are resiliently deformable along their length andallow the planar base 89, and so the peg 87 and the third wheel 80mounted on it, to move into the body 23 in the direction indicated byarrow B, on the application of a force on the planar base 89 in thedirection indicated by arrow B.

The stage 4 is configured such that when the first carriage 20 andsecond carriage 22 are assembled together the first 91, second 93, third95 and fourth 97 arms are deformed. Accordingly, due to theirresilience, the first 91, second 93, third 95 and fourth 97 arms willbias the third rigid wheel 80 into the third guide rod 72. This in turnwill cause the first 77 and second 79 rigid wheels to be biased into thefourth guide rod 78.

In the described embodiment, the stage 4 is configured so that the forceon the planar base 89 (caused by the force on the rigid third wheel 80by its engagement with the third guide rod 72) is greater than the yieldstress of the first 91, second 93, third 95 and fourth 97 arms.Accordingly, the first 91, second 93, third 95 and fourth 97 arms areplastically deformed on assembly of the first carriage 20 and secondcarriage 22.

Referring to FIGS. 6 a, 6 b and 7, a first drive unit 98 for driving thefirst carriage 20 relative to the sample positioning stage in the Xdimension, is mounted on the first carriage 20 adjacent the firstcarriage's third wheel 48 and frictionally engages the second guide rod38. A second drive unit 100 for driving the second carriage 22 relativeto the first carriage 20 in the Y dimension, is mounted on the secondcarriage 22 between the second carriage's first 77 and second 79 wheelsand frictionally engages the fourth guide rod 78. The first 98 andsecond 100 drive units are identical.

It has been found that providing the drive unit and the positionmeasurement devices on the side on which the rigid bearings are locatedimproves the accuracy and repeatability of the stage positioning. FIG.14 shows an embodiment of a stage 1004 which is substantially identicalto the stage 4 described in connection with FIGS. 1 to 13 and like itemsshare like reference numerals. However, rather than the first 98 driveunit which provides for relative movement between the plate 18 and thefirst carriage being on the side of the resiliently compliant bearing(not shown) it is located on the side of the rigid bearings (of whichonly the first rigid bearing 43 is visible—the second rigid bearingbeing obscured from view). As is the case with the stage 4 described inconnection with FIGS. 1 to 13, the second drive unit 100 which providesfor relative movement between the first carriage 20 the second carriage22 is still provided on the side of the rigid bearings (of which onlyfirst rigid bearing 83 is visible).

Referring to FIGS. 7 to 9, the second drive unit 100 comprises a motorbody 102 and a drive shaft 104 extending from the motor body 102 whichcan be rotated by the motor body 102. The motor body 102 is a 12 volt DCdirect drive motor. The motor body 102 receives power from a powersource (not shown) through electrical cables (not shown).

The drive shaft 104 has first 106 and second (not shown) conicalportions which each converge to a reduced diameter portion toward thecentre of the length of the drive shaft. When the sample positioningstage 4 is assembled, both of the first 106 and second conical portionscontact the fourth guide rod 78. Accordingly, the drive shaft 104 hastwo points of contact with the guide rod 78. As illustrated in FIGS. 15a to 15 c the first 105 and second 107 points of contact each lie on abearing contact plane illustrated by dashed line 109 which the containsthe points of contact between the second carriage's 22 first 77 andsecond 79 wheels and the fourth guide rod 78. In particular, the pointof contact 105 between the drive shaft 104 and the lower side of theguide rod 78 lies on a lower bearing contact line which contains thepoint of contact 111 between the first wheel 77 and the lower side ofthe guide rod and the point of contact 113 between the second wheel 79and the lower side of the guide rod 113. Furthermore, the point ofcontact 107 between the drive shaft 104 and the upper side of the guiderod 78 lies on an upper bearing contact line which contains the point ofcontact 115 between the first wheel 77 and the upper side of the guiderod and the point of contact (not shown) between the second wheel 79 andthe upper side of the guide rod 113.

The second drive unit 100 comprises a mounting arm 108. The motor body102 is secured to the mounting arm so that they cannot move relative toeach other. The mounting arm 108 has a base portion 110 to which themotor body 102 is secured by first 112 and second 114 screws, and an armportion 116 which extends away from the motor body 102 along the lengthof the fourth guide rod 78. The end of the arm portion 116 distal to themotor body 102 has an extension 118 which defines an aperture 120.

The mounting arm 108 is mounted on the second carriage 22 via a spring122. The spring 122 is secured to the second carriage 22 via a screw124, and to the mounting arm 108 by the end of the spring being hookedthrough the aperture 120. Accordingly, in this way the mounting arm 108is mounted on the second carriage so that the mounting arm 108, andhence the motor body 102 and drive shaft 104, is free to move relativeto the second carriage in all dimensions other than in the dimensiondefined by the rotational axis B of the drive shaft 104. Rotation of themounting arm 108, and hence the motor body 102 and drive shaft 104, isrestricted in a first direction by the spring 122 and in a seconddirection opposite to the first direction by the abutment of theextension 118 against the body 23 of the second carriage 22.

The second drive unit 100 further comprises a bias mechanism 126 forbiasing the drive shaft 104 onto the fourth guide rod 78. The biasmechanism 126 comprises a body 128 which has a head 130 and an arm 132,and a spring 144 which, when the sample positioning stage 4 isassembled, acts between the head 130 and the body 23 of the secondcarriage 22.

The head 130 has first 134, second 136, third 138 and fourth 140bearings. When the sample positioning stage 4 is assembled, the bearingsengage the drive shaft 104 and facilitate rotation of the drive shaft104 relative to the head 130.

A first end of the arm 132 is secured to the head 130, and the arm 132has an aperture 141 toward its second end which is distal to the head130. The bias mechanism 126 is coupled to the body 23 via a bolt 142which extends through the aperture 140 in the arm 132 and which engagesa threaded bore in the body 23. The arm 132 is flexible so that the head130 can be biased onto the second guide rod 78 by the spring 144.

To assemble the sample positioning stage 4, the first carriage 20 isslid into the plate 18 so that the first 44 and second 46 wheels engagethe first guide rod 34, and so that the third wheel 48, and the driveshaft 104 of the first drive unit 98 (which is mounted on the firstcarriage 20 in the manner described above) engage the second guide rod38.

The dimensions of the plate 18 and first carriage 20 are such that inorder for the first carriage 20 to be received within the plate 18, thefirst 84, second 86, third 88 and fourth 90 arms are deformed so thatthe third wheel 48 is compressed into the body 21 of the first carriage20. Furthermore, in the embodiment described the first 84, second 86,third 88 and fourth 90 arms are deformed to such an extent so as toplastically deform the first 84, second 86, third 88 and fourth 90 arms.Accordingly, when the first carriage 20 is received in the plate 18, thefirst 84, second 86, third 88 and fourth 90 arms bias the third wheel 48onto the second guide rod 38.

Furthermore, the spring 144 is compressed by the interaction between thedrive shaft 104 of the first drive unit 98 with the second guide rod 38of the plate 18, so that the drive shaft 104 is biased onto the secondguide rod 38.

The second carriage 22 is then slid into the first carriage 20 in amanner similar to that described above in relation to the first carriage20 and the plate 18.

The second carriage 22 is then fixed to the microscope 6 so that itcannot move relative to the objective lens 10 in the X and Y dimensions.Accordingly, the position of the plate 18 can be moved relative to theobjective lens 10, in the X dimension by operation of the first drivesystem 98, and in the Y dimension by operation of the second drivesystem.

A light source (not shown) can be positioned below the samplepositioning stage 4. Light from the light source can pass through theaperture 67 in the second carriage 22, the aperture 65 in the firstcarriage and the aperture 27 in the plate 18 so as to illuminate asample 8 located on the plate 18.

As shown in FIG. 11, the sample positioning stage 4 is connected to acontrol system 200 via an input/output line 202 (in cable 146). Thecontrol system 202, comprises an initialization module 204, a positionmaintenance module (PMM) 206 and a collision/drag detection module 208.The control system 200 is connected to an input device 210 viainput/output line 212.

The basic operation of the control system 200 will now be described inconnection with FIG. 12. The position maintenance module 206 begins atstep 300, when the control system 200 is first turned on. On startup,the position maintenance module 206 performs an initialization processat step 302. This involves calling a calibration routine from theinitialization module 204 which calibrates the sample positioning stage4. On completion of the calibration routine, the position maintenancemodule 206 receives the current position of the plate 18 relative to thefirst carriage 20 in the X dimension, and the current position of thefirst carriage 20 (and hence the plate 18) relative to the secondcarriage 22 in the Y dimension, and puts this data into a currentposition variable. Also as part of the initialization process 302, theposition maintenance module 206 sets up a demanded position variablewith its initial value being the same as the current position variable.

The demanded position variable can be changed by a user inputting a newdemanded position via the input device 210. In particular, the user caninput a demanded X position and a demanded Y position to the controlsystem via the input device 210. In the described embodiment, the userinputs an absolute demanded position (i.e. move the plate 18 to acertain X/Y position). However, it will be understood that the demandedposition input to the control system 200 can be a relative position(i.e. move the plate 18 in the X/Y dimension by a certain amount).

The position maintenance module continually monitors the output from thefirst 33 and second 37 readheads and updates the current positionvariable on detection of a change of position.

At step 304, the position maintenance module 206 continually checks tosee if the demanded position and the current position are the same. Ifnot, then at step 306, the position maintenance module applies a DCoutput voltage (“V”) across either, or both, of the motor bodies 102 ofthe first 98 and second 100 drive systems as required so as to move theplate 18 toward the demanded position. The output voltage V can beincreased up to a maximum output voltage so as to progress the plate 18towards the demanded position. As the plate 18 is moved, the currentposition variable is continually updated so as to reflect its currentactual position. This process continues until the current positionvariable is the same as the demanded position variable.

The user can manually drag the plate 18 by applying a force to the plate18 in the X and/or Y dimension. The user can apply such a force to theplate 18 by manipulating the handle 40.

The collision/drag detection module 208 runs in parallel to the positionmaintenance module 206, and is used to determine if the plate 18 isbeing manually dragged by the user, or if the plate 18 has collided withan object. If the collision/drag detection module 208 does detect such asituation, then it deactivates the motor bodies 102 of one of, or bothof, the first 98 and second 100 drive systems so as to stop them drivingagainst the external force. The operation of the collision/dragdetection module 208 is explained in connection with FIG. 13.

The collision/drag detection module 208 begins at step 500 when thecontrol system 200 is turned on. At step 502, the output voltage Vapplied by the position maintenance module 206 at step 306, iscontinually monitored to see if it is at its maximum level.

As can be seen from FIG. 12, when the plate 18 collides with an object,or is manually dragged away from the demanded position, the positionmaintenance module will attempt to oppose the collision/dragging byincreasing the output voltage (“V”) applied to the motor body 102 ofeither or both of the first 98 and second 100 drive units to full powerso as to bring the current position closer to the demanded position.Accordingly, the output voltage V will only be at its maximum level whenthe current position is not the same as the demanded position.

If the output voltage V is at is maximum voltage, then the processproceeds to step 504. At step 504, the position error is recorded in avariable PositionError0 and a timer (“t”) is started. The position erroris the difference between the demanded position and the currentposition.

Control proceeds to step 506 in which the timer is continuallyincremented until it has reached half its maximum value (“T”). In thedescribed embodiment, “T” is 25 mS. Accordingly, after approximately12.5 mS, it is determined if the output voltage V is still at is maximumlevel. If the output voltage V is not still at its maximum level thenthis means that the external force on the plate 18 has been removed andso the process restarts.

If the output V is still at its maximum level, then control proceeds tostep 508 at which the current position error is recorded in a variablePositionError1.

At 510, the timer t is continually incremented until it has reached itsmaximum value T. Accordingly, after approximately 25 mS from the startof the timer t, it is determined if the output voltage V is still at ismaximum level. If the output voltage V is not still at its maximum levelthen this means that the external force on the plate 18 has been removedand so the process restarts.

If the output voltage V is still at its maximum level, then controlproceeds to step 512 at which the current position error is recorded ina variable PositionError2.

At 514, it is determined if a force external to the plate 18 is beingapplied to it. If the sample positioning stage 4 has collided with anobject and stalled, then the position of the plate 18 between the timert being started and the timer t reaching the maximum number ofincrements T will not have changed significantly. Accordingly, if thedifference between the position in PositionError0 and the position inPositionError2 is smaller than a predetermined maximum movementthreshold X, then the method proceeds to step 518. The predeterminedmaximum movement threshold X enables a collision still to be detectedeven if the plate 18 moves a small distance between the recordal ofPositionError0 and PositionError2. In the described embodiment, thepredetermined threshold X is 100 μm.

If at step 514 the method does not determine that the plate 18 hascollided with an object, then the method proceeds to step 516 at whichit is determined if the plate 18 is being dragged manually.

At step 514, it is determined if either of the following are true:

0>PositionError0>PositionError1>PositionError2

0<PositionError0<PositionError1<PositionError2

These conditions are true whenever the plate 18 is being dragged awayfrom its demanded position. If either of these conditions are true, thenthe process proceeds to step 518. If neither of these conditions aretrue, then the process returns to step 502.

At step 518 the motor body 102 is turned off. This is done by thecollision/drag detection module 208 interrupting the positionmaintenance module 206 and reducing the DC output voltage V appliedacross the drive unit to zero. Accordingly, the motor body 102 will nolonger try to drive the plate 18 to the demanded position and the driveunit 100 can be easily backdriven by the user.

Control then proceeds to step 520 in which a timer Voff is started.Timer Voff has a maximum value of one second. At step 522, it isdetermined if the plate 18 is stationary. If the examination plate isnot stationary, then control returns to step 520, and the Voff timer isreset to zero.

If it is determined that the plate 18 is stationary, then controlproceeds to step 526 at which it is determined if Voff has reached itsmaximum value (i.e. one second). If not, then control returns back tostep 522 and the process is configured so that step 524 executes onceevery 200 μS after the timer Voff is started. If Voff has reached itsmaximum value, (i.e. one second) then control proceeds to step 528, atwhich point the demanded position variable is set as being the currentposition, and the position maintenance module 206 is restarted (fromstep 304).

The process described in relation to FIG. 13 is executed in connectionwith each of the first 98 and second 100 drive units independently.

As will be understood, other mechanisms other than the above describedmethod can be used to determine when to the turn the motors 100 and 104off so as to not to drive the plate 18 to a demanded position against anexternal force. For instance, a switch could be provided on the stage 4for enabling the user to switch the mode of operation between one modein which it drive tries to drive the plate 18 to a demanded position andanother mode in which it freely allows a user to backdrive the driveunits 98 and 100 so as to manually position the plate 18. Such a switchcould be provided, for example by a button on the stage 4. For example,the button could be placed in the location of the recess 42 on the plate18.

1. A sample positioning stage for positioning a sample to be inspectedrelative to an optical inspection device, the stage comprising: a firstgenerally planar body on which a sample to be inspected can be carried;a second body directly coupled to the first body via at least one rigidbearing and at least one resiliently compliant bearing provided betweenthe first and second bodies, the at least one rigid bearing and the atleast one resiliently compliant bearing being arranged generallyopposite each other, and configured such that the first and secondbodies are preloaded against each other in a dimension substantiallyparallel to the plane of the first generally planar body via thebearings, and such that movement of the first body relative to thesecond body is constrained to a first plane that is substantiallyparallel to the plane of the first body via the bearings; a motorizeddrive system operable to drive the first and second bodies relative toeach other in the first plane toward a demanded relative positionreceived from a position input device, in which the motorized drivesystem imparts its driving force on the stage closer to the at least onerigid bearing than the at least one resiliently compliant bearing; and aposition sensing device on at least one of the first and second bodiescloser to the at least one rigid bearing than the at least oneresiliently compliant bearing, for providing a measure of the relativeposition of the first and second bodies.
 2. A sample positioning stageas claimed in claim 1, in which the at least one rigid bearing comprisesat least one track on one of the first and second bodies, and at leastone runner on the other of the first and second bodies in bearingengagement with the track.
 3. A sample positioning stage as claimed inclaim 2, in which the motorized drive system acts on the track to causerelative movement.
 4. A sample positioning stage as claimed in claim 3,in which the motorized drive system acts on the bearing surface of thetrack.
 5. A sample positioning stage as claimed in claim 4, in which thedirection of the force applied by the motorized drive system on thetrack extends substantially parallel to a bearing contact line.
 6. Asample positioning stage as claimed in claim 4, in which the point atwhich the motorized drive system acts on the bearing surface of thetrack lies substantially on the bearing contact line.
 7. A samplepositioning stage as claimed in claim 1, in which the relative movementof the first and second bodies is constrained via the bearings to afirst degree of freedom which lies substantially parallel to the planeof the first body.
 8. A sample positioning stage as claimed in claim 7,comprising at least two rigid bearings spaced apart from each otheralong the first degree of freedom, and in which the motorized drivesystem acts on the first or second body at a point between the at leasttwo rigid bearings.
 9. A sample positioning stage as claimed in claim 1,in which the first and second bodies can move relative to each other ina linear dimension.
 10. A sample positioning stage as claimed in claim1, in which the first body is a plate and the second body is a carriage.11. A sample positioning stage as claimed in claim 2, in which the trackis a friction rod and the runner is a wheel.
 12. A sample positioningstage as claimed in claim 11, in which the motorized drive systemcomprises a motor body and a drive shaft extending from the motor body,the drive shaft being in frictional engagement with the friction rodsuch that rotation of the drive shaft causes the friction rod to moverelative to the drive shaft.
 13. A sample positioning stage as claimedin claim 1, in which the resiliently compliant bearing comprises abearing part which facilitates relative movement between the bodies anda resiliently compliant mounting which provides for resilientlycompliant movement of the bearing part.
 14. A sample positioning stageas claimed in claim 13, in which the resiliently compliant mountingbiases the bearings into bearing engagement.
 15. A sample positioningstage as claimed in claim 1, in which the position sensing devicecomprises a scale on one of the first and second bodies and a scalereader on the other of the first and second bodies, the scale and scalereader each being closer to the at least one rigid bearing than the atleast one resiliently compliant bearing.
 16. A sample positioning stageas claimed in claim 7, further comprising: a third body directly coupledto the second body via at least one rigid bearing and at least oneresiliently compliant bearing between the second and third bodies, theat least one rigid bearing and the at least one resiliently compliantbearing being arranged generally opposite each other and configured topreload the second and third bodies against each other so as toconstrain movement of the first body relative to the second body to asecond degree of freedom which lies in a plane that is substantiallyparallel to the plane of the first body and which extends substantiallyperpendicularly to the first degree of freedom; and a second motorizeddrive system configured to drive the first and second bodies relative toeach other along the second degree of freedom toward a demanded relativeposition received from a position input device, in which the motorizeddrive system imparts its driving force on the stage to cause therelative movement in the second degree of freedom closer to the at leastone rigid bearing than the at least one resiliently compliant bearing.17. A sample positioning stage as claimed in claim 16, furthercomprising a second position sensing device on at least one of thesecond and third bodies closer to the at least one rigid bearing thanthe at least one resiliently compliant bearing, for providing a measureof the relative position of the second and third bodies.
 18. An opticalinspection apparatus comprising: an optical inspection device; and asample positioning stage as claimed in claim 1 for positioning a sampleto be inspected relative to the optical inspection device.
 19. A samplepositioning stage for a microscope apparatus, comprising: a firstgenerally planar body having first and second linear trackssubstantially opposing and parallel to each other, at least the firstlinear track being rigidly fixed relative to the first body; a secondbody directly coupled to the first body so that it can move relative tothe first body in a degree of freedom, the second body having at leastfirst and second rigidly mounted runners in bearing engagement with thefirst linear track at points spaced along the first linear track so asto define the degree of freedom, the second body also having at leastone resiliently compliant runner in bearing engagement with the secondlinear track, in which the runners and tracks constrain movement of thefirst body relative to the second body to a first plane that issubstantially parallel to the plane of the first body; a motorized drivesystem fixed relative to the second body in the degree of freedom andconfigured to act on the first linear track at a point between which thefirst and second rigidly mounted runners engage the first linear trackso as to drive the first and second bodies relative to each other alongthe degree of freedom toward a demanded relative position received froma position input device; and a position sensing device on at least oneof the first and second bodies provided on the side of the bodies thatis closer to the rigidly mounted runners than the resiliently compliantrunner, for providing a measure of the relative position of the firstand second bodies.